There is known an image generation system for generating an image which is visible from a given viewpoint within an object space, that is, a virtual three-dimensional space. This is highly popular since players can experience a so-called virtual reality. For an image generation system by which a player can enjoy a gun game, the player can enjoy a three-dimensional game by using a gun-type controller (or shooting device) to shoot various target objects such as enemy characters (or computer objects) which are displayed on a screen.
It has been found that such an image generation system for gun game had one problem.
Such a problem is that if there is any obstruction is between a player (or a player's object) and an enemy character, a shot (or bullet) from the enemy character can pass through the obstruction and hit the player. This will render the player displeasing and impair the player's immersion in the game.
This problem may be overcome to some degree, for example, by using a certain technique of modifying the layout or moving route of the enemy characters. However, such a technique has a limitation.
Particularly, if the behavior of an enemy character cannot previously be determined, that is, if it will fully change depending on how the shot from the player hits the enemy character, the aforementioned technique cannot well work.